The invention relates to a liquid developer for developing positively charged electrostatic charge images. The developer is composed of an electrically insulating carrier liquid which has a high resistance and a low dielectric constant and which contains a pigment or dye, a resinous binder, a charge control substance and conventional additives in dispersion or solution. The invention is also directed to a charge control substance suitable in the developer.
In electrophotography, transfer processes have largely gained acceptance in practice in which the toner particles of the developer, deposited on the latent electrostatic charge image, are transferred from the photoconductor layer to a suitable receiving material, for example normal paper, and are fixed. The development of the charge image can be carried out with a dry developer or liquid developer.
Liquid developers are in principle composed of electrically insulating carrier liquids, in which pigments and dyes, resins, charge control substances and conventional additives are dispersed or dissolved. In the electric field of the charge image, the charged toner particles of the liquid developer are deposited electrophoretically on the charge image. For positive charge images, such as are formed, for example, in the electrophotographic process on photoconductive, positively charged selenium layers, liquid developers with negatively charged toner particles are required. To ensure a satisfactory transfer from the photoconductor layers to the receiving material, the toner particles must be coarser than in the case of non-transferable toners such as are required, for example, for the development of charge images on zinc oxide binder layers.
The negative charge of the toner particles of a liquid developer is obtained either by applying electronegative resins or polymers, which are sparingly soluble in the carrier liquid, to the surface of the toner or pigment particles, or by dissolving a soluble basic substance in the carrier liquid.
Numerous charge control agents have been disclosed which are soluble in the carrier liquid and effect negative charging. These include, for example, lecithin, alkaline earth metal salts of long-chain alkylbenzenesulfonic acids and dialkylsulfosuccinic acids, superbasic calcium alkylsulfonates or polymers with dialkylamino groups. Particularly suitable polymers are those which are soluble in the carrier liquid and which contain N-vinylpyrrolidone groups, for example, alkylated polyvinyl pyrrolidones (U.S. Pat. No. 3,542,682), copolymers of N-vinylpyrrolidone and long-chain methacrylates (EP-B No. 0,001,103) and graft copolymers obtained by grafting 1 to 20% of N-vinyl-pyrrolidone onto soluble homopolymers or copolymers of methacrylates (German Offenlegungsschrift No. 3,011,193).
Long-life developer liquids providing good copies are obtained in particular with the last mentioned graft copolymers. However, the developers do not yet meet the most stringent demands and require improvement. Depending on the copying speed and the blackening on the original to be copied, they contaminate the copying machines to different extents or the toner image is not completely transferred to the receiving material. The toner still present on the photoconductor layer after the passage through the transfer station can no longer be redispersed in an optimum manner, after it has been stripped off from the photoconductor layer in the cleaning station, and can collect in an undesirable way in the cleaning station.
In the case of unfavorable copying conditions, for example, after the copying machine has been at a stand-still for several days, the deposits can cause streaks on the copies to be made. The liquid developers themselves tend to gel on prolonged storage and are then unsuitable for controlled replenishment because they block the valves of the stock bottles.